1. Field of the Invention
The present invention relates to a mobile communication system for use in a cellular wireless communication system of digital mobile telephones, portable telephones, and the like.
2. Description of the Related Art
CDMA (Code Division Multiple Access) is one of the techniques for multiple access when a plurality of stations are simultaneously in communication in the same frequency band. There are other known techniques such as FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access), and compared with these techniques, CDMA is advantageous in that a frequency band is used more efficiently, and thus, that CDMA is available for more subscribers.
CDMA is a technique for carrying out multiple access utilizing spread spectrum communication where a spectrum of information signal is spread over a sufficiently wider band than the original bandwidth in transmission. Spread Spectrum-Direct Sequence (SS-DS) is a method of spreading a spectrum by directly multiplying an information signal by a spread code. Signals from a plurality of mobile stations are multiplexed in the same frequency band and in the same time period.
TDD (Time Division Duplex) is a system where the same band is used both for transmission and for reception, and is sometimes referred to as the ping-pong system. In TDD, the same radio frequency is time shared to transmission/reception to carry out communication. On the other hand, FDD (Frequency Division Duplex) is a system where a frequency used for transmission and a frequency used for reception are different in carrying out communication. FIG. 7A conceptually illustrates TDD, while FIG. 7B conceptually illustrates FDD. In FIG. 7A, at a time T1, a base station transmits data and a mobile station receives data, while, at the subsequent time T2, the mobile station transmits data and the base station receives data. By repeating the cycle, communication using the same frequency band is implemented. In FIG. 7B, in a frequency f1, a base station transmits data and a mobile station receives data, while, in a frequency f2, the mobile station transmits data and the base station receives data.
In SS-DS CDMA, there is a problem that, when a desired transmitting station is far away and an undesired transmitting station (interfering station) is nearby, the power of signals received from the interfering transmitting station is higher than that received from the desired transmitting station, and the mutual correlation between the spread codes can not be suppressed simply with the process gain (spread gain), leading to incapability of communication. This problem is "a problem of distance." Therefore, in an SS-DS CDMA cellular system, it is essential to control transmission power according to the status of the respective transmission lines in up circuits from mobile stations to a base station.
Also, as measured against fading, which is a cause of degradation of communication quality in terrestrial mobile communication, a method for compensating for instantaneous value fluctuation of reception power by controlling transmission power has been proposed.
Methods for controlling transmission power in CDMA/TDD are disclosed in theses entitled "REVIEW OF TRANSMISSION POWER CONTROL IN CDMA/TDD TRANSMISSION" (Miya, Hayashi, and Katoh, IEICE Spring Conference, 1994, B-418) and "POWER CONTROL IN PACKETS SWITCHED TIME DIVISION DUPLEX SEQUENCE SPREAD SPECTRUM COMMUNICATIONS" (R. Esmailzadeh, M. Nakagawa, and A. Kajiwara, Proc. of VTC '92, pp. 989-992, 1992). Fading is symmetrical in that fluctuation in transmission/reception is the same in a single frequency band. Therefore, CDMA/TDD is characterized in that, by using open loop transmission power control where the power of received signals is detected, the status of propagation is grasped, and a transmission power level is determined to transmit data, transmission power control can be carried out in a simpler way, at a higher speed, and more precisely, compared with FDD.
In CDMA, it is effective in increasing the capacity of communication to use codes of high orthogonality as spread codes. However, Walsh codes and orthogonal Gold codes, which are known as codes of high orthogonality are limited in their number to the same number as the code length. Therefore, in order to secure a sufficient number of spread codes to be allotted to the users, products of short codes the periods of which are the same as the symbol lengths of information by long codes the periods of which are longer than those of the short codes are used (U.S. Pat. No. 5,103,459). In this case, by making one base station used only one long code and by allotting different long codes to the respective base stations, the orthogonality of all users in one cell is secured. Further, since signals of other cells are spread using different long codes, these signals become noises. As a result, mutual interference is reduced.
In a cellular system, when a mobile station is switched on, or even when communication with a base station is disconnected, it is necessary to specify the nearest base station to carry out best communication. This is called judgment of the most preferable base station.
In a CDMA cellular system, in the judgment of the most preferable base station or when, according to movement of a mobile station during communication, a base station to which the communication is handed over is specified, conventionally, for example, all base stations transmit pilot channel data spread using the same spread code with offset phases (timing) so as to avoid alignment, and the mobile station correlatively detects all the phases of the spread code of received pilot channel data and specified as the nearest a base station transmitting data with a phase of the largest value of correlation (U.S. Pat. No. 4,901,307), or, base channels transmit perch channel data where different spread codes are allotted to the perch channel data of different base stations, and the mobile station receives perch channel data of all base stations and specifies the nearest base station by measuring the levels of the data.
In case judgment of the most preferable base station is made in a CDMA/TDD cellular system, a mobile station has to first acquire synchronization with respect to the TDD period (slot), specify the time of transmission by a base station, and acquire synchronization with respect to the spread code. Especially when another nearby mobile station is in communication, the transmission power of the nearby mobile station is dominant over that of signals to be received, which is an obstacle to acquisition of initial synchronization. However, up to now, no solution to this problem has been presented.
Further, in case product of a long code by a short code is used as a spread code, since it is also necessary to acquire synchronization with respect to the short code and the long code, time necessary to carry out synchronization and the judgment of the most preferable base station increases.
The present invention is made to solve the problems mentioned in the above, and it is an object of the present invention to provide a mobile communication system for a CDMA/TDD cellular system with long codes and short codes being used as spread codes which allows high-speed initial synchronization and judgment of the most preferable base station and high-speed acquisition of synchronization with respect to the long codes.